March 29, 2011 - BCAT-6 launched on space
shuttle Discovery's STS-133 mission on Feb. 24. Currently, BCAT-6
is a reserve payload and will be run in the future when time allows.
BCAT -3,-4 and -5 are presently on ISS: BCAT-3 is in storage awaiting
operations with the higher resolution 12 Mp Nikon Camera, BCAT-4 has
three (of ten) samples remaining in the baseline science matrix with
the 8Mp Kodak Camera. BCAT-5 launched on 2 J/A (June 13, 2009)
and is awaiting approval to operate in the JEM.
The BCAT-6 investigation is structured from a rich
history of space flight experiments that explore the fundamental physical
science and application of colloids in a microgravity environment.
Colloids are a type of homogeneous mixture in which very small particles
of one substance are distributed evenly throughout another substance.
Paints, milk, fog, butter, smoke, ink, paint are colloids. The BCAT-6
series hardware consists of the same design as that used for BCAT-4
and BCAT-5. This effort will address fundamental questions in colloidal
engineering that impact product shelf life and determine how concentrated
systems of particles of select sizes and shapes cause order to naturally
arise out of disorder when gravity is removed.
The BCAT-5 experiment has started operations on the
International Space Station (ISS). It contains experiments from five
teams of scientists in a collaborative effort with the Canadian Space
Agency (CSA), and is the first stand-alone experiment to be run in
the Japanese Experiment Module (JEM) on the ISS.
Binary Colloidal Alloy Test-4 (BCAT-4) is a fluids experiment with
two parts: BCAT-4-CP and BCAT-4-Poly.
The BCAT-4-CP part of the experiment from Harvard University (David
Weitz and Peter Lu) and Simon Fraser University (Barbara Frisken and
Arthur Bailey) will measure phase separation rates and properties
of a model critical fluid system. Acquiring this data should
lead to a much better understanding of the shelf-life of products
and how to extend it. This portion of this microgravity experiment
will be accomplished by photographing the time evolution of seven
critical point (CP) samples, which will add needed points to the phase
diagram outlined by the related critical point samples in the BCAT-3
The Binary Colloidal Alloy Test-3 (BCAT-3) hardware supported three
investigations in which ISS crews photographed samples of colloidal
particles (tiny nanoscale spheres suspended in liquid) to document
liquid/gas phase changes, growth of binary crystals, and the formation
of colloidal crystals confined to a surface. Colloids are small enough
that in a microgravity environment without sedimentation and convection,
they behave much as atoms and so can be used to model all sorts of
phenomena because their size, shape, and interactions can be controlled.
The Binary Colloidal Alloy Test-3 is an Exploration
Systems' transition flight experiment in the Human System Research
and Technology area. BCAT-3 provides a unique opportunity to explore
fundamental physics and simultaneously develop important future technology,
including computers operating on light, complex biomolecular pharmaceuticals,
clean sources of geothermal power, and novel rocket engines for interplanetary
travel. These studies depend entirely on the microgravity environment
provided by the International Space Station (ISS); in all other locations
accessible to science, gravity dominates and precludes investigation
of any other effects of interest. The experiment itself is simple
and elegant, photographing samples of colloidal particles with a digital
camera onboard the ISS. Colloids are tiny nanoscale spheres of plexiglass
a thousand times smaller than the width of a human hair (submicron
radius) that are suspended in a fluid. They are ubiquitous (e.g.,
milk, smoke, and paint) and therefore interesting to study directly.
Colloids are also small enough that they behave much like atoms and
so can be used to model all sorts of phenomena because their size,
shape, and interactions can be controlled. The 10 samples in BCAT-3
are made from the same ingredients, each a recipe with different proportions,
and are grouped into three experiments: critical point, binary alloy,
and surface crystallization.
In an ordinary pot of boiling water, bubbles of water
vapor coalesce on the bottom of the pot, growing until they detach
and float to the surface where they escape into the atmosphere. At
the boiling temperature water exists simultaneously in two distinct
phases—liquid and gas—and as the bubbles burst, those
two phases are spatially separated. But what should the mixture look
like in the absence of gravity, when the vapor no longer floats to
the top? Moreover, the behavior changes with increasing pressure:
seal the pot, as in a pressure cooker, and the boiling temperature
rises. Continuing the pressure increase, the mixture will reach its
critical point, a unique pressure and temperature value where the
properties of liquid and gas merge. Just above is the supercritical
regime where they are no longer distinct phases, but rather a homogeneous
supercritical fluid. The BCAT-3 samples of David Weitz and Peter Lu
of Harvard University model this behavior in a colloidal system, where
the phases analogous to liquid and gas can be seen as two different
Supercritical fluids are technologically important because they uniquely
combine the properties of liquids and gases, flowing easily (like
gases), yet still having tremendous power to transport dissolved materials
and thermal energy (like liquids). Supercritical carbon dioxide is
used to decaffeinate coffee beans and to extract complex biomolecules
from plants for pharmaceutical research. Supercritical water so efficiently
transports heat that it is being explored in Iceland as a potentially
superior geothermal power source; it is also used to remove toxic
waste from contaminated soil. Additionally, NASA's Jet Propulsion
Laboratory is working on using supercritical fluids as unique propellants
for future rocket engines. A better understanding of critical behavior
as a result of microgravity experiments like BCAT-3 might thus facilitate
the development of new drugs, cleaner power, and interplanetary transportation.
The BCAT-3 critical point samples may also have tremendous impact
upon fundamental physics. Understanding critical phenomena was an
important theoretical advance in physics during the last half century,
but ground-based experiments have been limited by gravity, which invariably
causes the denser liquid phase to fall to the bottom of any container,
precluding direct observation of phase separation alone. While previous
colloidal experiments have tantalizingly shown that the boundary between
separating phases in the absence of gravity is like a jagged coastline,
BCAT-3 is the first to systematically attempt to precisely locate
the critical point and visualize the behavior around it.
Colloids are also technologically interesting because
they are the rightsize to manipulate light. Natural opal is likely
the oldest and best known of the "photonic" crystals that
direct light: shine white light on the opal and a rainbow appears,
demonstrating how colors of light are split up and sent in different
directions. The ability to better control the movement of light is
a major technological goal, not only to build computers operating
on light instead of electricity, but also to harness the full capabilities
of existing fiber-optic networks for improving communications. Crystal
structures built from only one building block—such as the arrangement
of colloidal silica spheres in an opal—are well under-stood,
but their optical properties are limited. More useful photonic crystals
can be built from two different types of building blocks mixed together,
yielding a binary alloy. The resulting structures and their optical
properties are vast, as both the size and proportion of the two building
blocks can be varied. How the crystallization is affected by these
changes is only beginning to be explored. Theoretical studies suggest
that desired optical properties require more complicated crystal structures,
but this has not been well explored experimentally. Microgravity is
crucial to the binary crystal experiments, allowing the growth of
crystals far larger than those created on the surface of the Earth.
The BCAT-3 binary alloy sample of Peter Pusey and Andy Schofield of
the University of Edinburgh furthers previous investigations on binary
growth in space.
Crystal structures are affected not only by constituent
building blocks, but also by the geometrical environment where they
grow. The long, thin blades of ice on the surface of a freezing puddle
are far different from both the solid blocks in a freezer ice cube
tray and the six-sided needles in a snowflake. Arjun Yodh and Jian
Zhang at the University of Pennsylvania have prepared several samples
to study the formation of colloidal crystals confined to a surface,
allowing comparison with bulk three-dimensional crystallization, to
begin teasing out how geometry affects crystallization itself.
The Next Step
BCAT-3 is a simple experiment with both important
technological applications and profound implications for fundamental
physics. The phenomena under investigation require an environment
where gravity plays no significant role, and this can only be done
in space. However, BCAT-3 is limited by the nature of macroscopic
photography: the camera cannot resolve individual colloidal particles.
The more sophisticated Light Microscopy Module (LMM) scheduled to
fly to the ISS in 2006 is a microscope designed to view and locate
the millions of particles in these samples one by one, further enabling
and deepening our understanding. Understanding supercritical fluids
may accelerate present efforts to use them as propellants in advanced
rocket engines. Developing more efficient optical communications through
better photonic crystals is particularly important to onboard computers
in outer space, where electronic circuits are constantly being degraded
by high-energy cosmic rays; optical circuits are immune to such wear.
Those developments may be critical for long, interplanetary missions.
In summary, BCAT-3 provides a unique opportunity to understand fundamental
physics and develop important future technologies— all in a
package the size of an ordinary textbook.
Image above: Astronaut Leroy
Chiao works on the BCAT-3 experiment on the International Space Station.